Add set_frequency method on PWM channels

[azerupi]

Following discussion in #172 this adds a method to PWM channels to change the underlying timer's frequency. This allows to dynamically change the PWM frequency when we have already associated a channel with a pin. Otherwise this required unsafe to work around a partial move error because the channel was moved out of the timer struct.

In the implementation I changed clock_frequency on the Instance to not take &self. This allows it to be used in the channels. But this is not required, we could access the RCC register directly to avoid making a breaking change?

Compared to the timer's implementation we don't stop and start the timer before changing the frequency. I tested it on my project and it seems to work flawlessly. I'm not sure why that was required in the timer's implementation? If it needs to be added, is there a way to reuse the timer's implementation without copy-pasting?

Let me know if anything can be improved! 🙂

[azerupi]

The clippy failure is the same as what I got in #173, it has been fixed there.

[hannobraun]

Thank you, @azerupi!

[azerupi]

Great, thanks!

Just to make sure, you did see this part?

In the implementation I changed clock_frequency on the Instance to not take &self.

Because I think that is a breaking change since Instance is public. 🙂

[dbrgn]

Breaking changes are fine, we have breaking changes all the time. However, maybe you could send a PR that updates the CHANGELOG?

[hannobraun]

I did see that, but thanks for pointing it out again. I could have been a bit less terse 🙂

While it is strictly speaking a breaking change, I don't think there's much reason for end users to touch Instance except in trait bounds. Its methods are more of an internal API, although Rust won't let us express that. So I think this change has pretty low impact, and given that, I believe that using unsafe to conjure up an Rcc is not worth it in this case.

Also, what @dbrgn said, we break things all the time 😄 In a perfect world, we'd take more steps to avoid/alleviate that, but we have to work with the resources we've got (on the maintainer and contributor side).